Re: [PATCH net,v5] virtio_net: clamp rss_max_key_size to NETDEV_RSS_KEY_LEN

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Mar 26, 2026 at 07:53:44PM +0530, Srujana Challa wrote:
> rss_max_key_size in the virtio spec is the maximum key size supported by
> the device, not a mandatory size the driver must use. Also the value 40
> is a spec minimum, not a spec maximum.
> 
> The current code rejects RSS and can fail probe when the device reports a
> larger rss_max_key_size than the driver buffer limit. Instead, clamp the
> effective key length to min(device rss_max_key_size, NETDEV_RSS_KEY_LEN)
> and keep RSS enabled.
> 
> This keeps probe working on devices that advertise larger maximum key sizes
> while respecting the netdev RSS key buffer size limit.
> 
> Fixes: 3f7d9c1964fc ("virtio_net: Add hash_key_length check")
> Cc: stable@vger.kernel.org
> Signed-off-by: Srujana Challa <schalla@marvell.com>


Acked-by: Michael S. Tsirkin <mst@redhat.com>

> ---
> v3:
> - Moved RSS key validation checks to virtnet_validate.
> - Add fixes: tag and CC -stable
> v4:
> - Use NETDEV_RSS_KEY_LEN instead of type_max for the maximum rss key size.
> v5:
> - Interpret rss_max_key_size as a maximum and clamp it to NETDEV_RSS_KEY_LEN.
> - Do not disable RSS/HASH_REPORT when device rss_max_key_size exceeds NETDEV_RSS_KEY_LEN.
> - Drop the separate patch that replaced the runtime check with BUILD_BUG_ON.
> 
>  drivers/net/virtio_net.c | 20 +++++++++-----------
>  1 file changed, 9 insertions(+), 11 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 022f60728721..b241c8dbb4e1 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -373,8 +373,6 @@ struct receive_queue {
>  	struct xdp_buff **xsk_buffs;
>  };
>  
> -#define VIRTIO_NET_RSS_MAX_KEY_SIZE     40
> -
>  /* Control VQ buffers: protected by the rtnl lock */
>  struct control_buf {
>  	struct virtio_net_ctrl_hdr hdr;
> @@ -478,7 +476,7 @@ struct virtnet_info {
>  
>  	/* Must be last as it ends in a flexible-array member. */
>  	TRAILING_OVERLAP(struct virtio_net_rss_config_trailer, rss_trailer, hash_key_data,
> -		u8 rss_hash_key_data[VIRTIO_NET_RSS_MAX_KEY_SIZE];
> +		u8 rss_hash_key_data[NETDEV_RSS_KEY_LEN];
>  	);
>  };
>  static_assert(offsetof(struct virtnet_info, rss_trailer.hash_key_data) ==
> @@ -6717,6 +6715,7 @@ static int virtnet_probe(struct virtio_device *vdev)
>  	struct virtnet_info *vi;
>  	u16 max_queue_pairs;
>  	int mtu = 0;
> +	u16 key_sz;
>  
>  	/* Find if host supports multiqueue/rss virtio_net device */
>  	max_queue_pairs = 1;
> @@ -6851,14 +6850,13 @@ static int virtnet_probe(struct virtio_device *vdev)
>  	}
>  
>  	if (vi->has_rss || vi->has_rss_hash_report) {
> -		vi->rss_key_size =
> -			virtio_cread8(vdev, offsetof(struct virtio_net_config, rss_max_key_size));
> -		if (vi->rss_key_size > VIRTIO_NET_RSS_MAX_KEY_SIZE) {
> -			dev_err(&vdev->dev, "rss_max_key_size=%u exceeds the limit %u.\n",
> -				vi->rss_key_size, VIRTIO_NET_RSS_MAX_KEY_SIZE);
> -			err = -EINVAL;
> -			goto free;
> -		}
> +		key_sz = virtio_cread8(vdev, offsetof(struct virtio_net_config, rss_max_key_size));
> +
> +		vi->rss_key_size = min_t(u16, key_sz, NETDEV_RSS_KEY_LEN);
> +		if (key_sz > vi->rss_key_size)
> +			dev_warn(&vdev->dev,
> +				 "rss_max_key_size=%u exceeds driver limit %u, clamping\n",
> +				 key_sz, vi->rss_key_size);
>  
>  		vi->rss_hash_types_supported =
>  		    virtio_cread32(vdev, offsetof(struct virtio_net_config, supported_hash_types));
> -- 
> 2.25.1